1. Field of the Invention
The present invention relates to a DDFSE (Delayed Decision Feedback Sequence Estimator) for estimating a transmission signal from a signal having undergone transmission path distortion caused by frequency selective fading due to a multipath effect in a radio channel in high-speed digital communication and, more particularly, to a delayed decision feedback sequence estimation diversity receiver which improves its signal estimation ability by combining antenna diversity with a DDFSE.
2. Description of the Prior Art
As a conventional apparatus designed to determine an optimal reception timing so as to estimate a transmission signal from reception signals having undergone transmission path distortion by using a DDFSE, the delayed decision feedback sequence estimation receiver disclosed in Japanese Unexamined Patent Publication No. 11-8573 is known.
The DDFSE is a signal estimator which has the merits of both an MLSE (Maximum Likelihood Sequence Estimator) having high signal estimation ability and a DFE (Decision Feedback Equalizer) with a small computation amount.
FIG. 1 is a block diagram showing the arrangement of a conventional DDFSE with a timing control function.
Assume that a reception signal 201 is a complex baseband signal expressed in a two-dimensional form. A transmission path estimator 202 is a block for obtaining the characteristics of a transmission path in the form of an impulse response. In general, the transmitting side sends a training signal before transmission of data, and the receiving side receives the training signal having undergone transmission path distortion, thereby obtaining transmission path characteristics.
An estimation region detector 203 performs a computation to find the timing at which the signal estimation ability is maximized. A DDFSE 204 performs signal estimation on the basis of the impulse response sequence obtained by the transmission path estimator 202 and the optimal timing obtained by the estimation region detector 203.
If the impulse response sequence obtained by the transmission path estimator 202 has undergone transmission path distortion, it has a temporally wide waveform like the one shown in FIG. 2. In this case, this signal is expressed in the form of a discrete signal sampled at a symbol period T of the transmission signal. FIG. 2 shows how the distortion spreads over a time 6T (signal components a2, a3, and a6 to a10 are not shown because their amplitudes are regarded as 0).
Assume that the DDFSE with the timing control function is configured to perform transmission path estimation in 11 symbol periods. More specifically, the DDFSE performs signal estimation equivalent to an MLSE computation in the first three symbol periods, and cancels a component corresponding to the succeeding three symbol periods by a computation equivalent to a DFE computation.
The estimation region detector 203 can find an optimal timing by the following computation.
Let P be the power component used for signal estimation, which falls within a 3-symbol range (MLSE region), Q be the power component to be canceled, which falls within a 3-symbol range (DFE region), and R be the power in the remaining 5-symbol range (outside the estimation region). In this case, as P increases, the signal estimation ability increases. Q is irrelevant to the signal estimation ability because it is canceled. As R increases, the signal estimation ability decreases. As an evaluation function, we define:Z=P/R  (1)
The signal estimation ability is maximized at the timing at which Z of equation (1) is maximized.
In general, an impulse response in a transmission path can be obtained accurately only within certain limits on the receiving side owing to the influences of noise and computation errors. For this reason, the signal component in the DFE region which should be completely canceled ideally is not completely canceled and left as a distortion component. This phenomenon becomes noticeable as the signal component in the MLSE region decreases and the signal component in the DFE region increases.
A decision feedback loop exists in the DDFSE. Once an error is made in signal estimation, therefore, the erroneous estimation result circulates within the loop, and a burst-like error called error propagation may occur. This error propagation is likely to occur as the component in the DFE region becomes large. In order to cope with this situation, the evaluation function expressed by equation (1) must be modified to determine the timing at which higher signal estimation ability can be obtained. To this end, we define an evaluation function given by:Z=P/(R+αQ)  (2)
In equation (2), the coefficient α is a coefficient determined in accordance with the computation precision of an impulse response.
In the transmission path impulse response sequence shown in FIG. 2., the timings represented by:P=(a0)2+(a1)2+(a2)2  (3)Q=(a3)2+(a4)2+(a5)2  (4)R=(a6)2+(a1)2+(a8)2+(a9)2+(a10)2  (5)are obtained as optimal timings for signal estimation by using either equation (1) or (2).
If signal components that are received with delays are larger than other components as shown in FIG. 3, the timings obtained by equations (1) and (2) may differ from each other. In using equation (2), the timings are matched to delayed components that are received with delays by adjusting the coefficient a as per:P=(a3)2+(a4)2+(a5)2  (6)Q=(a6)2+(a7)2+(a8)2  (7)R=(a9)2+(a10)2+(a0)2+(a1)2+(a2)2  (8)
This is because the estimation ability can be improved by performing signal estimation using a4 and a5 while regarding a0 and a1 as distortion components rather than by performing signal estimation using a0 and a1 with small amplitudes.
FIG. 4 shows the arrangement of this estimation region detector 203.
A power calculator 701 obtains the power level of each symbol, which is the square value (the sum of the square value of a real part and the square value of an imaginary part) of each symbol, of the complex impulse response sequence output from the transmission path estimator 202, and inputs the respective power levels to shift registers 702a to 702j. 
An adder 703 obtains a power value P of the signal component in the MLSE region. An adder 704 obtains a power value Q of the signal component in the DFE region. An adder 705 obtains a power value R of a signal component outside the estimation region for the DDFSE 204.
Equations (3) and (6) are calculated by the adder 703. Equations (4) and (7) are calculated by the adder 704. Equations (5) and (8) are calculated by the adder 705.
The power values P, Q, and R obtained by the adders 703, 704, and 705 are used by an evaluation function calculator 706 to perform a computation based on equation (2). The evaluation function calculator 706 calculates equation (2) over 11 symbol periods, and detects the timing at which the value of Z is maximized. The evaluation function calculator 706 then outputs this timing to the DDFSE 204.
In this manner, the DDFSE with the timing control function obtains the timing for signal estimation by using an evaluation function like equation (2), thereby obtaining an optimal timing for the DDFSE.
However, the following problem arises in the prior art described above.
In a transmission path impulse response sequence like the one shown in FIG. 3, if a0 and a1 are received in the MLSE region as optimal timings, a4 and a5 received in the DFE region are canceled by a0 and a1 having small amplitudes. At this time, if a slight error is included in a0 or a1, the error is amplified when a4 and a5 are canceled, resulting in a deterioration in signal estimation ability.
If the values of a4 and a5 are large, the probability of occurrence of error propagation, i.e., continuous occurrence of errors upon occurrence of an error in signal estimation, increases. This also leads to a deterioration in signal estimation ability.
If a4 and a5 are received in the MLSE region, since a0 and a1 are received in neither the MLSE region nor the DFE region, these values are not effectively used for signal estimation and treated as distortions. This becomes a factor that degrades the signal estimation ability. That is, high signal estimation ability can be obtained by selecting neither of the former timing and the latter timing.
When a relatively large power component is set in the DFE region, as shown in FIG. 2, error propagation occurs more easily than when a large power component is not set in the DFE region. Therefore, a deterioration in signal estimation ability cannot be avoided.